With the increasing down-scaling of integrated circuits and increasingly demanding requirement to the speed of integrated circuits, transistors need to have higher drive currents with smaller dimensions. Various methods have been developed to increase the drive currents of transistors. For example, Fin Field-Effect Transistors (FinFETs) and Gate All-Around (GAA) transistors were developed. FinFETs and GAA transistors have increased channel widths. For a FinFET, the increase in the channel widths is achieved by forming channels that include portions on the sidewalls of a semiconductor fin and a portion on the top surface of the semiconductor fin. For a GAA transistor, the channel is formed around a semiconductor region. Since the drive currents of transistors are proportional to the channel widths, the drive currents of the FinFETs and GAA transistors are increased over that of conventional planar transistors.
Another widely used method for increasing the drive currents includes applying stresses to the channel regions of transistors. For example, stresses may be applied by forming source/drain stressors in the source/drain regions of the transistors. Other available methods include forming a strained layer, such as a strained contact etch stop layer to cover the transistor. Additional methods are also explored.